Thermoelectric Performance Of Bi-Sb-Te Based Compounds

Thermoelectric materials can realize the direct conversion between thermal energy and electricity.The Seebeck effect can be used for thermoelectric power generation,and the Peltier effect can be used for refrigeration and heating.Thus,thermoelectric materials show great potential in waste heat recovery and electronic devices cooling.Bi₂Te₃based compounds,which have excellent thermoelectric properties around room temperature,have been used in commercial refrigeration devices.The thermoelectric properties of Bi₂Te₃based bulk materials can be greatly improved by doping,solid solution,nanostructuring,and other methods.As a mature commercial thermoelectric material,it needs a shorter production cycle and a lower preparation cost.Due to the intrinsic lattice defects of Bi₂Te₃based compounds,the carrier concentration is sensitive to doped elements.Therefore,how to utilize and control these defects to achieve better thermoelectric properties in the desired temperature range becomes very important.In order to improve the thermoelectric properties and reduce the cost of Bi₂Te₃based thermoelectric materials,a systematic study on Bi Sb Te based solid solution was carried out.The achievements are summarized as follows:1.Cu-doped p-type Bi_0.5Sb_1.5Te₃compounds have been prepared by a facile hydrothermal method,which not only shortens the preparation time of the sample,but also effectively reduces the preparation cost by replacing the metal with metal salt.With the addition of Cu,the greatly raised hole carrier concentration effectively suppresses the bipolar effect.The optimum working temperature range of materials moves towards higher temperature.Furthermore,the small grain size prepared by hydrothermal method greatly decreased lattice thermal conductivity,which is beneficial for improving the thermoelectric properties.A dimensionless thermoelectric figure of merit?z T?value of 1.2 at room temperature is achieved for the sample with 0.6 wt%Cu?nominal composition?,and the z T value increases with the temperature to 1.5 at 150°C.The Cu-doped Bi_0.5Sb_1.5Te₃sample prepared by hydrothermal method shows excellent thermoelectric performance and has great potential as a thermoelectric material near room temperature.2.Because the working temperature of?Bi,Sb?₂Te₃based thermoelectric materials is generally around room temperature.Improving the room-temperature thermoelectric performance of p-type?Bi,Sb?₂Te₃is the key to its practical application.However,the usual doping or alloying methods increases the carrier concentration and leads to an increase in thermoelectric performance at high temperatures rather than at room temperature.In this work,we find that Ti is a promising dopant to shift the optimum thermoelectric performance of p-type?Bi,Sb?₂Te₃to room temperature by reducing its carrier concentration.p-type Bi_0.5Sb_1.5-xTi_xTe₃samples with various Ti contents have been prepared using a simple melting method.The carrier concentration of Bi_0.5Sb_1.5-xTi_xTe₃is reduced by partially replacing Sb with Ti,leading to not only a significantly increased Seebeck coefficient but also an improved power factor near room temperature.Moreover,the total thermal conductivity near room temperature also decreases owing to the combined effect of decreased electrical conductivity and an anisotropic microstructure.An optimal z T value of?1.2 is achieved near room temperature for the sample containing 6 at%Ti,and its average z T value below 150°C increases to?1.1,demonstrating the great potential of this material for room-temperature thermoelectric devices.3.The thermoelectric performance of p-type Bi_0.5Sb_1.5Te₃was adjusted by electro-thermal-magnetic coupling,proving that magnetic doping is an effective strategy in Bi_0.5Sb_1.5Te₃system.Ferromagnetic Fe/Co and diamagnetic Pb have been doped into p-type Bi_0.5Sb_1.5Te₃to investigate the effect of different magnetic dopants on the electrical and thermal transport properties.According to our experiments,it is much more difficult for Fe/Co to enter the Bi_0.5Sb_1.5Te₃lattice in comparison with Pb,which can be understood by the‘like dissolves like'rule.At the same doping content,Fe and Co provide much lower hole carriers than Pb due to their larger carrier thermal activation energies,indicating that Fe and Co as dopants are very applicable for fine regulation of carrier concentration.The Fe/Co doped samples have higher Seebeck coefficients but less carrier mobilities than the Pb doped sample since the ferromagnetic doped element induces additional carrier scattering mechanism.Beyond the solid solubility limit,excess Fe/Co presents as the impurity,which can maintain a high carrier concentration due to the metal-semiconductor contact.Finally,the optimal z T values of?1.1 and 1.15 at 100°C have been achieved for the samples with 1.71 at%Co and 1.80 at%Fe,respectively.4.Since bismuth telluride-based alloys have intrinsic lattice defects,reasonable control of these defects will help improve the performance of the material.For p-type Bi₂Te₃-based compounds,maximum thermoelectric figure of merits z Ts are often shifted to higher temperature due to the increased carrier concentration,but at the cost of reducing mobility.Herein,we report the simultaneously increased carrier concentration and mobility in p-type Bi_0.5Sb_1.5Te₃through Cd doping.The doping of Cd successfully suppresses the bipolar diffusion due to the significantly increased hole concentration.Meanwhile,the Hall mobility increases with the doping of Cd owing to the decrease of anti-site defects.In addition,the lattice disorder induced by the doping of Cd leads to an obvious reduction in lattice thermal conductivity.As a result,a z T value of 1.2 at 120°C is achieved for the 2 at%Cd doped sample.The peak z T of the 4 at%Cd doped sample is shifted to 185°C due to the effectively suppressed bipolar effect and an average z T value of?1.0 is obtained in the temperature range of 50-300°C.5.The lattice point defects and anti-site defects were regulated by replacing Te with a small amount of Se in Bi_0.5Sb_1.5Te₃.When the content of Se is low,the concentration of Bi'_Seanti-site defect increases due to the volatilization of some Se,and the carrier concentration increases with the increase of Se content,thus improving the electrical conductivity.When the Se content continues to increase,part of the Se enters the lattice to replace Te,and the anti-site defects is suppressed due to the increase in the difference in electronegativity between Se and the cation.At the same time,the addition of Se increases the density of states effective mass,resulting in an increase in the Seebeck coefficient,which ultimately increases the power factor of 2 at%Se doped sample.In addition,the anti-site defects introduced by Se volatilization or the point defects introduced by Se into the lattice instead of Te enhance the phonon scattering,resulting in a significant decrease in the lattice thermal conductivity.Finally,for 2 at%Se doped samples,the z T value reaches 1.4 at 100?.In the temperature range of 50-300?,the z T values of Se doped samples are higher than that of undoped samples.The results of this study show that there is still a lot of room for improving the thermoelectric properties of Bi Sb Te materials through the regulation of lattice defects.